Smart Antenna Systems
Antenna arrays may be used in any wireless communication receiver or transmitter or transceiver (herein under “communication station”) that transmits or receives radio frequency signals using an antenna or antennas. The use of antenna arrays in such a communication station provides for antenna performance improvements over the use of a single element antenna. These antenna performance improvements include improved directionality, signal to noise ratio, and interference rejection for received signals, and improved directionality, security, and reduced transmit power requirements for transmitted signals. Antenna arrays may be used for signal reception only, for signal transmission only, or for both signal reception and transmission.
A typical application of antenna array communication stations is in a wireless communication system. Examples include a cellular communication system consisting of one or more communication stations, generally called base stations, each communicating with its subscriber units, also called remote terminals and handsets. In cellular systems, the remote terminal may be mobile or in a fixed location, and when fixed, such a system often is called a wireless local loop system. The antenna array typically is at the base station. Terminology for the direction of communication comes from conventional satellite communication, with the satellite replaced by the base station. Thus, communication from the remote terminal to the base station is called the uplink, and communication from the base station to the remote terminal is called the downlink. Thus, the base station antenna array transmits on the downlink direction and receives on the uplink direction. Antenna arrays also may be used in wireless communication systems to add spatial division multiple access (SDMA) capability, which is the ability to communicate with several users at a time over the same “conventional” (FDMA, TDMA or CDMA) channel. We have previously disclosed adaptive smart antenna processing (including spatial processing) with antenna arrays to increase the spectrum efficiency of SDMA and non-SDMA systems. See Co-owned U.S. Pat. No. 5,515,378 for SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEM, U.S. Pat. No. 5,592,490 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, U.S. Pat. No. 5,828,658 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, and U.S. patent application Ser. No. 08/729,390 for METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING. Systems that use antenna arrays to improve the efficiency of communications and/or to provide SDMA sometimes are called smart antenna systems.
With smart antenna communication systems that use linear spatial processing for the adaptive smart antenna processing, during uplink communications, one applies amplitude and phase adjustments in baseband to each of the signals received at the antenna array elements to select (i.e., preferentially receive) the signals of interest while minimizing any signals or noise not of interest—that is, the interference. Such baseband amplitude and phase adjustment can be described by a complex valued weight, the receive weight, and the receive weights for all elements of the array can be described by a complex valued vector, the receive weight vector. Similarly, the downlink signal is processed by adjusting the amplitude and phase of the baseband signals that are transmitted by each of the antennas of the antenna array. Such amplitude and phase control can be described by a complex valued weight, the transmit weight, and the weights for all elements of the array by a complex valued vector, the transmit weight vector. In some systems, the receive (and/or transmit) weights include temporal processing, and then are called spatio-temporal parameters for spatio-temporal processing. In such cases, the receive (and/or transmit) weights may be functions of frequency and applied in the frequency domain or, equivalently, functions of time applied as convolution kernels. Alternatively, each convolution kernel, if for sampled signals, may itself be described by a set of complex numbers, so that the vector of convolution kernels may be re-written as a complex values weight vector, which, for the case of there being M antennas and each kernel having K entries, would be a vector of KM entries.
The receive spatial signature characterizes how the base station array receives signals from a particular subscriber unit in the absence of any interference or other subscriber units. A receive weight vector for a particular user may be determined using different techniques. For example, it may be determined from spatial signatures. It also may be determined from the uplink signals received at the antennas of the array from that remote user using some knowledge about these uplink signals, for example, the type of modulation used. The transmit spatial signature of a particular user characterizes how the remote user receives signals from the base station in the absence of any interference. The transmit weight vector used to communicate on the downlink with a particular user is determined either from the receive weight vector (see below under “The Need for Calibration”) or from the transmit spatial signature of the particular user and the transmit spatial signatures of the other users in such a way as to maximize the energy to the particular user and minimize the energy to the other users.
U.S. Pat. No. 5,592,490 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS describes spatial signatures and their uses, and U.S. Pat. No. 5,828,658 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, incorporated herein by reference, describes how to extend this to spatio-temporal processing using spatio-temporal signatures.
Thus, while the description herein is provided in terms of spatial signatures, adding time equalization to provide spatio-temporal processing is easily accommodated, for example by adding the concepts of spatio-temporal signatures, which may be described by MK vectors (both uplink and downlink) when the temporal processing is using equalizers with K taps (i.e., convolution kernels of length K in the weight convolving functions). Thus, how to modify the invention to accommodate spatio-temporal processing and spatio-temporal signatures would be clear to those of ordinary skill in the art, for example in view of above-referenced and incorporated herein by reference U.S. Pat. No. 5,828,658. Therefore, those in the art would understand that any time the term spatial signature is used, this might indeed be referring to a spatio-temporal signature in the context that the invention is being applied to a communication station equipped with means for spatio-temporal processing.
The Need for Calibration
It is desirable to determine the transmit weight vector from the receive weight vector for a particular user. More generally, it is desirable to determine the appropriate transmit signals to use for transmitting to a particular user from signals received from that user. Practical problems may make difficult determining the transmit weight vector from the receive weight vector for a particular user. Frequency division duplex (FDD) systems are those in which uplink and downlink communications with a particular remote user occur at the different frequencies. Time division duplex (TDD) systems are those in which uplink and downlink communications with a particular remote user occur at the same frequency but in different time slots. In a TDD system, because of the well known principle of reciprocity, it might be expected that determining the transmit weight vector from the receive weight vector is straightforward. However, on the uplink, the received signals that are being processed may be somewhat distorted by the receive electronics (the receive apparatus chains) associated with each of the antenna elements of the antenna array. The receive electronics chain includes the antenna element, cables, filters, RF receivers and other components, physical connections, and analog-to-digital converter (“ADC”) if processing is digital. In the case of a multi-element antenna array, there typically is a separate receive electronics apparatus chain for each antenna array element, and thus the amplitude and the phase of each of the received signals at each element may be distorted differently by each of the receive apparatus chains. In addition, there are RF propagation effects that take place on the uplink between the subscriber unit and a particular receiving antenna, such effects including without limitation the path loss, fading and shading effects, multipath, and near-field scattering, and these effects may be different from antenna element to antenna element. Note that the receive electronics chain and the RF propagation effects together make up the uplink spatial signature for the remote user. A receive weight vector that does not take these receive electronics chain and RF propagation effects into account will be in error, causing less than optimal reception at the base station. However, in practice, communication may still be possible. Also, when a receive weight vector is determined using some knowledge of the characteristics of the received signal, for example, the type of modulation used, such a method already takes into account the uplink receive electronics chain and RF propagation effects. When one transmits downlink signals through the antenna array, each of the signals radiated by an antenna element goes through a different transmit electronics apparatus chain; thus possibly causing different amplitude and phase shifts in the transmitted signals. In addition, there are again RF propagation effects. If the transmit weight vector was derived from a receive weight vector that did not take the differences in the receive electronics chains and RF propagation into account, transmission from the base station may be hard to achieve. Further difficulty may result if the transmit weight vector does not take differences in the transmit electronics chains and transmit RF propagation effects into account, possibly making communication using such a transmit weight vector impossible.
The purpose of calibration is to determine calibration factors for compensating for the different amplitude and phase errors that occur in the signals in the receive chain and uplink RF propagation, and the different amplitude and phase errors that occur in the transmit chain and downlink RF propagation, the calibration factors used in a communication station to determine a transmit weight vector for transmitting to a remote user from the set of signals received from the remote user. It should be added that because the phase and amplitude shifts that occur in the receive and transmit apparatus chains are, in general, frequency dependent, so in general are the calibration factors frequency dependent.
In the case of a TDD system, the uplink and downlink RF propagation effects cancel so that the calibration factors are independent of the location of the subscriber unit.
It is known that compensation can be achieved by convolving each of the M signals received or transmitted by the antenna elements by a calibration function (i.e., by a complex valued time sequence), where each calibration function describes the transfer function correction required to compensate for the gain and phase errors a signal undergoes when passing through the transmit and receive apparatus chains. In some systems, this can be simplified to multiplicative correction, where each calibration function is a calibration factor—a complex valued number that describes the required amplitude and phase correction required for compensation. In general, the set of calibration functions defines a calibration vector function with each element a calibration function. In the case of multiplicative correction, the set of calibration factors defines a calibration vector with each element a calibration factor.
Determining the transmit weight vectors from the receive weight vectors for a particular user is more difficult in the case of an FDD system because reciprocity may no longer be assumed. One needs to additionally take into account the differences in propagation on the uplink and downlink. Once one does take such differences into account, there still is a need to determine calibration factors for compensating for the different amplitude and phase errors that occur in the signals in the receive chain and uplink RF propagation and the different amplitude and phase errors that occur in the transmit chain and downlink RF propagation. In general, single calibration factors that are independent of the location of the remote user may not be possible. In such a case, one needs to be able to determine the uplink and downlink spatial signatures.
In the case of no calibration factors that are independent of the remote user location being possible, when there is some functional relationship that enables one to determine the transmit weight vector to use from the received signals and some parameter, for example, the angle of arrival, there still is a need to determine a set of calibration functions for compensating for the different amplitude and phase errors that occur in the signals in the receive chain and uplink RF propagation and the different amplitude and phase errors that occur in the transmit chain and downlink RF propagation, these functions being dependent on one or more parameters of the remote user, for example the angle of arrival.
The Need for Signature Estimation
When no simple calibration (as defined above) is possible, one still needs to compensate for the different amplitude and phase errors that occur in the signals in the receive chain and uplink RF propagation, and the different amplitude and phase errors that occur in the transmit chain and downlink RF propagation. The purpose of signature estimation is to determine the uplink and downlink spatial signatures which characterize these differences. Thus calibration is a special case of signature estimation when either 1) the RF propagation effects cancel so that downlink weights can be determined from uplink signals or weights, or 2) there is some simple functional relationship of the RF propagation effects so that uplink weights can be determined from uplink signals and some parameters of the remote user, for example, the angle of arrival of the uplink signals.
Other Methods
Known methods for determining array calibrations each have one or more associated drawbacks. Most known methods require external measuring equipment which may be expensive, unwieldy and cumbersome to use repeatedly. Secondly, conventional calibration methods are sensitive to drifts in system parameters, such as frequency references, over the extended period of time during which measurements are being taken, and these drifts result in inaccuracies in the measured array calibrations. In addition, some known techniques only determine multiplicative rather than convolution kernel calibrations despite the need to calibrate frequency dependent components in the antenna array. In order to eliminate this frequency dependence and still use multiplicative calibrations, it is necessary to calibrate the antenna array for each frequency channel of communication. Thirdly, the transfer characteristics of the RF electronics depend on changing ambient conditions such as temperature and humidity which make it essential that antenna arrays be repeatedly calibrated in their ambient environment.
Harrison et al. disclose in U.S. Pat. No. 5,274,844 (Dec. 28, 1993) a method for calibrating transmit and, separately, receive chains (as complex valued vector transfer functions) in two experiments which involve a data bus connecting a resource controller to a remote terminal. In the first experiment, the data bus indicates to the remote terminal to send a known signal to the base station. This determines the receive apparatus chain calibration. In a second experiment, the signals received at the remote terminal are sent back to the resource controller via the data bus to enable determining the transmit apparatus chain calibration.
Co-owned U.S. Pat. No. 5,546,090, issued Aug. 13, 1996, and assigned to the assignee of the present invention, discloses a calibration method which can determine both transmit and receive calibrations using a simple transponder co-located with the remote terminal that retransmits to the base station the signals received at the remote terminal from the base station. Such a method does not require the wired data-bus of the Harrison et al. invention. Still, additional transponder equipment is required.
PCT Patent application publication WO 95/34103 (published Dec. 14, 1995) entitled ANTENNA ARRAY CALIBRATION, Johannisson, et al., inventors, discloses a method and apparatus for calibrating the transmission (and reception) of an antenna array. For transmit calibration, an input transmit signal is inputted into each antenna element one antenna at a time. After the input transmit signal has passed through a respective power amplifier, the signal transmitted by each antenna element is sampled by a calibration network. The resulting signal is fed into a receiver, and a computation means relates the received signal with the original transmit signal for each antenna element. Correction factors can then be formed for each antenna element. The antenna elements may then be adjusted (in amplitude and phase, or in-phase I and quadrature Q components) using the correction factors so as to ensure that each element is properly calibrated during transmission. For receive calibration, a known input signal is generated and injected using a calibration network (a passive distribution network) into each antenna element of the antenna array. The signals pass from the antenna elements through respective low noise amplifiers, and the signals thus received by each antenna element are measured by a beam forming apparatus. The beam forming apparatus can then generate correction factors by comparing the injected signal with the measured signals so as to individually calibrate each antenna element. The correction can be described as amplitude and phase corrections, or as corrections in in-phase I and quadrature Q components.
U.S. Pat. No. 5,530,449 to Wachs et al. entitled PHASED ARRAY ANTENNA MANAGEMENT SYSTEM AND CALIBRATION METHOD (herein under “Wachs”) describes a management system and calibration method for use with a phased array antenna that employs a system level measurement of amplitude and phase, conducted during nodal operation, to determine on an element by element basis, the tracking performance of individual chains for the antennas. The system and method measure the amplitude and phase of individual element chains utilizing probe carriers. The required correction coefficients for each chain are determined from the measured amplitude and phase data, and each individual element chain is individually compensated to remedy the amplitude and phase errors. The system separately calibrates forward and return link phased array antennas on a phased array antenna communication station which is on a satellite. In one embodiment, a separate remote calibration station is used. For calibrating the transmit paths, the probe signal is transmitted to an antenna at the calibration system alternatively from one element (a reference element) and an element under test. The signals received at the calibration station are compared to determine the corrections. A separate communication link also is used to provide communication between the calibration station and the satellite. In the receive direction, the remote calibration station is used to transmit to all antenna elements of the phased array, but only two elements are alternately sampled to form the calibration carrier. The calibration carrier is then downlinked at Ka band to a gateway hub station for computation. In an alternate embodiment, a local sense antenna at the satellite's communication station is used to sample outputs of the transmit antenna elements. In both embodiments, separate calibrations are carried out for receive and transmit paths, and extra equipment is needed, either a separate remote calibration station, with an additional link, or a separate sense antenna system. Several features of Wachs' system are of note. First, additional hardware is required in the form of a separate calibration station or probe antenna. Second, special waveforms need to be used for that calibration, rather than ordinary communication waveforms supported by standard air interfaces. This means that the communication station needs additional hardware for forming and transmitting such waveforms, and the calibration station needs special receiving/demodulating hardware, and cannot reuse standard hardware. Thus there is a chance that a Wachs-like system adapted for use in a wireless communication system may not be allowed to operate in some countries.
Thus these known methods provide separate calibrations for the receive and transmit paths. The methods require special calibration apparatus. Some known methods and systems use special waveforms, and thus need additional hardware for processing such waveforms, and also do not conform to any established air interface standards, so face the risk of not being allowed to operate in some countries Those known systems that also calibrate for the different air paths between the base station antenna elements and the subscriber unit are more properly classified as spatial signature estimating techniques under the definition of calibration used herein.
Parish et al. in co-owned U.S. patent application Ser. No. 08/948,772 for METHOD AND APPARATUS FOR CALIBRATING A WIRELESS COMMUNICATION STATION HAVING AN ANTENNA ARRAY, describe a calibration method for a base station with an array of antenna elements that does not require any additional calibration apparatus. One aspect includes transmitting a prescribed signal from each antenna element using the transmit electronics of that antenna element while receiving the transmitted signal in at least one of the receiver electronics chains not associated with the antenna. This is repeated, transmitting prescribed signals from other antenna elements using other transmit apparatus chains until prescribed signals have been transmitted from all antenna elements for which calibration factors are required. Calibration factors for each antenna element are determined as a function of the associated transmit electronics chain and receiver electronics chain transfer functions. When downlink and uplink communication occurs in the same frequency channel, a single calibration factor is determined for any antenna element. In one version of the Parish et al. invention, the single calibration factor is in phase a function of the difference between the transmit apparatus chain transfer function phase and the receiver apparatus chain transfer function phase associated with a particular antenna element. In another aspect of the Parish et al. invention, the calibration factors so determined are used for determining a set of transmit weights from a set of receive weights.
While the Parish et al. invention enables determining a single set of calibration factors for the base station which enables a downlink set of weights to be determined from an uplink set of weights without requiring some additional apparatus such as a transponder, and calibrates for differences in base station electronics paths, the Parish et al. method cannot be adapted to estimate spatial signatures to deal with RF propagation path differences which may occur. In addition, the base station needs to enter a spatial calibration mode for carrying out the calibration experiment, and thus cannot be used for any other purpose during that time.
Also, there is no mention in the prior art of the capability of calibrating by combining measurements from a plurality of remote transceivers.
Desirable Features
The main purpose of the calibration process is to acquire calibration information for the base station. This may involve measuring the gain and phase differences between the uplink and downlink channels. Accuracy and high precision are of great importance during this procedure. If the calibration information is not accurate, then the beam pattern on the downlink will be highly distorted. As a consequence, less energy will be radiated toward the target user, and an excess amount of interference will be radiated toward co-channel users. This will have a negative effect on the downlink signal quality and on the downlink range. Ultimately, a bad calibration strategy may significantly reduce the capacity of the wireless network.
One desirable feature of a calibration method is that only a base station and a subscriber unit are needed for calibration with no further equipment such as signal generators, transponders, calibration stations, additional antennas, probes, or other equipment, being required. Such a system ideally should be able to calibrate for differences in both the receive and transmit electronics. Such systems also should use ordinary communication waveforms substantially conforming to the particular air interface standard of the wireless communication system in which they operate. This enables reusing standard hardware, and also ensures non-violation of standards and maintaining compatibility with any future modifications with standards. By “conforming to an air interface standard” we mean conforming to the channel structure and modulation of an air interface, where “channel structure” is a frequency slot in the case of FDMA, a time and frequency slot in the case of TDMA, and a code channel in the case of CDMA, and “modulation” is the particular modulation scheme specified in the standard.
Another desirable feature is that the method can be used for signature estimation in order to also account for differences in the RF paths.
Another desirable feature of a calibration method is ease of use and the ability to carry out the calibration rapidly and frequently, even for example, as frequently as several times a minute. This ultimately increases the downlink processing accuracy which has a profound effect on signal quality, capacity, coverage, and possibly other parameters.
Another desirable feature of a calibration method is that each and every subscriber unit supports calibration.
Another desirable feature for a calibration system is the ability to carry out some or all of the processing of received data for calibration within the subscriber unit, thus not requiring the subscriber unit to send the received data back to the base station and not requiring the base station to carry out all of the processing. The computational burden of the base station thus may be significantly reduced by “distributing” the load across intelligent subscriber units. This feature is particularly desirable, for example, for base stations that service many subscriber units, or that calibrate before each call or even several times during each call.
Another desirable feature is the ability to initiate calibration on any available conventional channel on the base station, for example, any carrier and any time slot of a FDMA/TDMA system. This further enhances flexibility since one can choose any timeslot and any carrier which is available for use at the moment.
Another desirable characteristic for a calibration method is the ability to calibrate a base station without having to take the base station off-line for calibration, thus enabling base station calibration to be performed while the base station services hundreds of calls, for example, in a FDMA/TDMA/SDMA system on other carriers (frequency slots)/timeslots/spatial channels. This feature is especially important for wideband base stations that service many conventional channels (e.g., carriers for an FDMA/TDMA system) at the same time.
Another desirable characteristic for a calibration method is the ability carry out rapid calibration even several times during an existing call.
Another desirable characteristic for a calibration method is the ability to carry out calibration in a seamless manner during an ongoing call so that a base station may be able to continuously calibrate itself during some calls.
Another desirable characteristic for a calibration method is the ability to carry out calibration with several remote transceivers by combining measurements, each of which may be able to “see” only a subset of a communication station's antenna array, or each of which may face a different interference environment.
Another desirable characteristic for a calibration method is the ability to determine whether calibration is accurate, for example by performing statistical measurements, together with the ability to feed back such information to the communication station to determine, for example, if the combining from several remote stations may be necessary.
Another desirable feature is high accuracy, with immunity to frequency offset, timing misalignment, I/Q mismatch, and phase noise that typically might occur in communication with inexpensive subscriber units.
Thus there still is a need in the art for a calibration method and apparatus that include all or most of the above characteristics. For example, the is a need for a system and method one that are accurate and simple, both in terms of the equipment necessary and the time required, so that calibration can be performed repeatedly and rapidly wherever and whenever desired. There also is a need in the art for a simple calibration technique that only uses existing base station electronics and does not require special calibration hardware. There also is a need in the art for a method that enables one to determine transmit weight vectors from receive weight vectors, including calibrating for the receive electronics and transmit electronics, the calibration obtained using simple techniques that use existing base station and subscriber unit electronics and do not require special calibration hardware.
Thus there still is a need in the art for efficient methods that determine uplink spatial signatures for correcting for the differences in uplink RF paths and receive electronics and downlink spatial signatures for correcting for the differences in downlink RF paths and transmit electronics.